I just did the transition from Mobile Me to iCloud. First I switched one family e-mail address to iCloud. Then I made sure that one other had already migrated and confirmed that the other addresses were unused.

I finally clicked the buttons and tested as follows:

I clicked the suggested buttons on one Mac and my iPhone.

I added a calendar event on one Mac and saw that it updated without prompting on another Mac, and my iPhone.

I sent an e-mail on my .mac account to my Mac, and it worked. (I do this often to transfer data from my iPhone to my iMac.)

I made a test change in Address Book and saw that it was reflected in my iPhone. (And erased it and lo, saw that it was erased.)

Any other suggestions for testing before I assume everything just worked?

And, why was there so much angst about the transition to iCloud? (Note that I'm only dealing with one Apple ID.)

"Design is not just what it looks like and feels like. Design is how it works."
Steve Jobs

After looking at this a while, I think it's still all or nothing in Photostream. It doesn't look like you can designate only certain files if Photostream is on. It's all recent photos up to 1000. (Of course, you can sync a folder of photos from Aperture or iPhoto by way of iTunes.)
So I don't think I can do what I want: iPhone photos wirelessly synced to Aperture library. Nothing going the other way except what I designate in iTunes.

(For me, the last 1000 photos would be really boring, mostly multiple shots of insects...)

I'm also unclear on what you can do with iTunes and iCloud. Apparently if you buy music on your iPhone, it'll sync to iTunes on your Mac (via iCloud). And, for a fee, you can have your whole library (up to a point) in the cloud.

But if you want, say, a rotating set of songs synced to the iPhone, iCloud doesn't come into it. You just do that through the normal playlist and sync that playlist.

"Design is not just what it looks like and feels like. Design is how it works."
Steve Jobs